Fix system email issues from cPanel Plesk server running CentOS

We rent a dedicated CentOS v5.5 server, running Plesk 11.0.9 and php 5.3.3 on which we have several websites. However, we are unable to receive any website system generated email. For example if an order is placed on one site, it should email us, we don't receive this email. On another site if a quote form is made, we should receive a copy of this quote. On another site if a registration form is filled in we should get the email. We need this fixing please!